feat(lib): Add warn

lib/nix-lib/nixpkgs.nix

@@ -413,4 +413,50 @@ rec {
   flip =
     f: a: b:
     f b a;
+
+  /**
+    `warn` *`message`* *`value`*
+
+    Print a warning before returning the second argument.
+
+    See [`builtins.warn`](https://nix.dev/manual/nix/latest/language/builtins.html#builtins-warn) (Nix >= 2.23).
+    On older versions, the Nix 2.23 behavior is emulated with [`builtins.trace`](https://nix.dev/manual/nix/latest/language/builtins.html#builtins-warn), including the [`NIX_ABORT_ON_WARN`](https://nix.dev/manual/nix/latest/command-ref/conf-file#conf-abort-on-warn) behavior, but not the `nix.conf` setting or command line option.
+
+    # Inputs
+
+    *`message`* (String)
+
+    : Warning message to print before evaluating *`value`*.
+
+    *`value`* (any value)
+
+    : Value to return as-is.
+
+    # Type
+
+    ```
+    String -> a -> a
+    ```
+  */
+  warn =
+    # Since Nix 2.23, https://github.com/NixOS/nix/pull/10592
+    builtins.warn or (
+      let
+        mustAbort = builtins.elem (builtins.getEnv "NIX_ABORT_ON_WARN") [
+          "1"
+          "true"
+          "yes"
+        ];
+      in
+      # Do not eta reduce v, so that we have the same strictness as `builtins.warn`.
+      msg: v:
+      # `builtins.warn` requires a string message, so we enforce that in our implementation, so that callers aren't accidentally incompatible with newer Nix versions.
+      assert builtins.isString msg;
+      if mustAbort then
+        builtins.trace "␛[1;31mevaluation warning:␛[0m ${msg}" (
+          abort "NIX_ABORT_ON_WARN=true; warnings are treated as unrecoverable errors."
+        )
+      else
+        builtins.trace "␛[1;35mevaluation warning:␛[0m ${msg}" v
+    );
 }